Corona cell



Oct. 1, 1963 K. K. KONRAD 3,105,918

CORONA CELL Filed April 27, 1961 INVENTOR KENNETH K.KONRAD "HMMEK \cQwL A TTORNEV State QIQilt 3 ,165,918 Patented Get. 1, 1963 3,105,918 CORONA CELL Kenneth K. Konrad, St. Albans, W. Va, assignor to Union Carbide Corporation, a corporation of New York Filed Apr. 27, 1961, Ser. No. 105,977 4 Claims. ('61. 313-231) The present invention relates to a corona cell for use in gas stream analyzer apparatus employing corona discharge to effect analysis. Such a gas stream analyzer and method is the invention of and the subject of a patent application of L. I. Rogers, US. application Serial No. 106,062, filed April 27, 1961. According to an analytical method predicated upon the corona discharge principle, trace quantities in very small percent by volume concentration of gases, vapors and liquids in continuously flowing streams can be readily and continuously detected and quantitatively measured. Corona cells according to the present invention are especially useful in corona type analyzers used in gas chromatography.

Dielectric breakdown in gases occurs in several different forms which are customarily classified as coronas, glow discharges, sparks and arcs. Corona occurs when the potential of a conductor is raised to a value at which the dielectric strength of the surrounding medium is exceeded. Corona manifests itself by a colored aura, the color depending upon the constituency of the medium surrounding the conductor, which appears around the conductor. In air, for example, corona appears as bluish tufts or streamers around the conductor, being more or less concentrated at irregularities on the conductor surface. It is known that corona is due to ionization of the conductor surrounding medium and that this ionized medium forms a high resistance conductor which increases the effective diameter of the metallic conductor. Ions are repelled from and drawn to the conductor at high velocity, producing other ions by collision. It follows that if a fluid, a gas or gas mixture other than the medium originally surrounding such a conductor is introduced into a corona aura, it too will be ionized by high velocity collisions, effecting changes in ionic concentrations, in which, in turn, effect changes in the resistance presented by the corona.

While the foregoing brief discussion of corona is sufficient for an understanding of the present invention, an extensive discusion of the corona phenomena will be found in Peek, Dielectric Phenomena in High Voltage Engineering, 3rd ed., 1929, McGraw-I-lill Book Company, Inc., New York City, New York.

It has been discovered that the impedance changes in corona occasioned by ionization changes afford a highly sensitive and linear signal source for measurement of trace quantities of gases, vapors and liquids in the low parts per million ranges and the present invention relates to a cell to contain corona discharges and to electrodes in said cell.

The apparatus of my invention comprises a corona cell uniquely suitable for use in a corona type gas analyzer which analyzer employs a radio-frequency power source and current detector means adapted to sense impedance changes in the cell. The invention also comprises novel details of construction and novel combinations of components, together with other features and results which will be more apparent from the following description. The drawings merely show and the description merely describes prefered embodiments of the present invention which are given by way of illustration.

In the drawings:

FIGURE 1 is an elevational view of the corona cell of the present invention and FIGURE 2 is a central sectional view of the cell of FIGURE 1.

Referring to the drawings, a cell body 11 is seen to comprise a U-shaped main duct 13 and an electrode housing 21. A central bore extends through the U- shaped main duct 13, defining a connecting passage 15 between a cell inlet 17 and a cell outlet 19. Inlet 17 and outlet 19 are adapted for connection into an external conduit system containing the stream to be processed through the analyzer of which the apparatus of the present invention is a component. Midway along the U-shaped main duct 13, disposed at right angles thereto, is an electrode housing 21 having a passage 23 extending therethrough and intersecting passage 15 in the main duct 13. The intersecting passages bisect the electrode housing 21 defining two portions. An electrode bushing 25 is installed in each portion of housing 21 and elec trodes 27 are coaxially disposed in the bushings. Each electrode is provided with threads 29 for adjustment of the electrode spacing and a generally conically formed tip 31. End plates 33 provided with threaded holes to engage the threads 29 of the electrodes, are arranged over the respective bushings and secured to a yoke 35 which is drilled and tapped to accommodate screws 37. In addition to securing the cell assembly, the end plates provide a current path from the electrodes to external circuitry. Locknuts 39 are provided on the threads 29 of the electrodes 27 to secure the electrodes when they have been adjusted to a desired spacing. A Wire connector 41 is provided under a head of each screw 37 to facilitate wiring the cell to other components of the analyzer.

' -In the preferred embodiment, the cell body 11 comprising the U-shaped main duct and the electrode housing is fabricated from glass. Cell bodies have been made from acrylic resin materials such as methyl methacrylate plastic and from aluminum. Plastic cells tended to break down atfer about two days of operation, presumably because :of high local temperatures developed around the corona aura. The aluminum cell had good sensitivity but caused sporadic noise signalsin the analyzer when the corona randomly touched the cell Walls. Glass has been found most suitable because it does not break down in continued use, corona discharge can be observed while the cell is in operation, agglomerations of impurities on the electrode tips can be detected and glass cells are easily cleaned.

The electrode bushings 25 may be made from any material which is chemically compatible (-i.e., non-reactive) with the streams analyzed. If a metal such as aluminum is used for the cell body, the bushings must be of electrically non-conductive material to avoid short circuiting the electrodes. In the preferred embodiment, the bushings are formed from a plastic consisting of a tetrafinoroethylene polymer. End plates 33, since they provide a path for cell current, must be formed from an electrically conductive substance. In the embodiment shown in the drawings the end plates are of brass. Yoke 35 can be made from any suitable non-conductive material such as an acrylic resin plastic or the like.

The electrodes 27 can be of any electrically conductive material. Both magnetic and non-magnetic metals have been used to fabricate the electnodes and it has been found that most satisfactory operation of the cell obtains with non-magnetic electrodes made from one of the group consisting of chromium, gold, silver, bronze, copper, molybdenum and the non-magnetic alloys of iron and nickel and with electrodes of non-magnetic alloys having gold tips. Gold makes the most satisfactory electrode tips since it is substantially inert and is not affected by oxidation impurities as are certain other metals. In experiments to determine the most effective electrode ti-p shapes, several configurations were tested and the conical tips 31, as illustrated in FIGURE 2 of the drawings, were found to develop the most stable corona and the lowest noise level for preselected values of frequency and potential at the analyzer power source. Further experimental tests were performed to determine optimum conical tip apex angles and it was found that relatively large 'apex angles at the conical tips efiected more stable corona, lower noise levels and decreased sensitivity, While relatively small angles effected increased sensitivity, some increase in noise level wd decreased stability. Data collected during the experiments indicates that electrode tips shaped to an apex angle of from about 30 to about 60 have substantially the same operating characteristics. In the preferred embodiment of apparatus according to my invention, the electrodes are made from non-magnetic stainless steel with conically formed go ld tips having apex angles of about 45. Inter-tip spacing for the electrodes required to produce corona, will of course, depend upon the values selected for other parameters of the malzer in which the cell of the present invention is used.

In the preferred embodiment, the electrodes 27 are disposed oppositely at substantially right angles to the passage 15, and are adjustable with respect to interelectrode spacing by means of threads 29. Since any stream passing through the cell will fill all void spaces therein, the electrodes are said to project into the stream or into passage even though for a particular analytical procedure the electrodes may be retracted to a point at which interelectrode spacing exceeds the nominal crosssectional dimensions of the passage through the cell."

In a typical application, a corona cell according to the present invention is connected in circuit with a radiofrequency power source and a current detector comprised in a vacuum tube circuit adapted to function as a radiofrequency oscillator and a current detector. The cell is capacitively coupled to a grid of the vacuum tube and variations in the cell impedance, caused by changes in the ionic concentration occurring in the corona aura, present a variable load to the oscillator output signals. Loading of the grid to which the cell is coupled is thus proportional to cell conductance which depends upon the ionized media within the corona, and grid loading variations cause proportional changes in plate current through the tube. Plate current variations are detected and applied as input signals to a potentiometer strip chart recorder which is calibrated in units of gas or vapor concentration.

A compact and easily maintained corona cell has been provided in accordance with the present invention. The cell is especially suitable for use in conjunction with gas stream analyzers employing coronasdischarge to ionize streams analyzed. While the novel features of the invention as applied to a preferred embodiment have been electrodes made from one of the group consisting of chromium, bronze, gold, silver, molybdenum, copper and the nonmagnetic alloys of iron and nickel, appositely disposed in said cell body and having respective conically formed tips each having an apex angle of from about 30 to about projecting into said passage; and means for connecting said electrodes to electrical circuitry exterior of said cell.

2. Apparatus according to claim 1 in which each of said respective conically formed tips has an apex angle of about 45 3. A corona cell comprising, in combination, a cell body; a passage extending through said body connecting an inlet and an outlet, said inlet and said outlet being adapted for respective connection in a conduit system exterior of the cell; a pair of electrically separated electrodes appositely disposed in said cell body having respective conically formed tips projecting into said passage, said electrodes being of non-magnetic stainless steel with conic-ally formed gold tips having respective conical apex angles of from about 30 to about 60; and means for connecting said electrodes to electrical circuitry cxterior of said cell.

4. Apparatus according to claim 3 in which each of said respective conically formed tips has an apex angle of about 45 References Cited in the file of this patent UNITED STATES PATENTS 2,640,870 Seitz June 2, 1953 2,753,479 Aughey et al. July 3, 1956 2,929,953 Mitteldorf et al. Mar. 22, 1960 3,025,745 Liston Mar. 20, 1962 

1. A CORONA CELL COMPRISING, IN COMBINATION, A CELL BODY; A PASSAGE EXTENDING THROUGH SAID CELL BODY CONNECTING AN INLET AND AN OUTLET, SAID INLET AND SAID OUTLET BEING ADAPTED FOR RESPECTIVE CONNECTION IN A CONDUIT SYSTEM EXTERIOR OF THE CELL; A PAIR OF ELECTRICALLY SEPARATED ELECTRODES MADE FROM ONE OF THE GROUP CONSISTING OF CHROMIUM, BRONZE, GOLD, SILVER, MOLYBDENUM, COPPER AND 